[mythtv-users] HDHomerun signal strength required

Brian Wood beww at beww.org
Fri Feb 22 15:20:42 UTC 2008


George Mari wrote:
> backuppc at sundquist.imapmail.org wrote:
> [deleted]
> 
>> Sounds like my situation, although my antennas are now in the attic.  My
>> RG-6 run from the merger to my FE/BE box is 50 feet, so I may consider
>> an amplifier if the HDHR and tweaking the antenna aiming doesn't improve
>> my signals.  Any suggestions for decent quality amplifiers (keeping
>> price in mind)?
>>
> 
> The pre-amps from Channel Master are well-regarded, models 7775, 7777, 
> and 7778.  They apparently generate much less noise than most other 
> brands, making them more effective.  I have a 7777 myself, and can vouch 
> for it's effectiveness.  They can be had for around $50.

Those are fairly good amplifiers, especially for the price.

The problem with such amps is that they really want all the channels 
going through them to be at more or less the same level. If you have one 
or more strong local channels and a few weak ones the level of the local 
stations will cause severe distortion products in the amplifier, leading 
to the "making things worse" situation.

One solution is to use separate antennas for each channel, designed to 
receive that channel only and attenuate others. This solution is used by 
CATV systems but is not usually practical for the home user.

Another solution is to use an adjustable trap to attenuate the level of 
the local station(s). Obviously this has to be installed ahead of the 
amplifier. The problem is that to properly adjust the trap you have to 
have a signal level meter at the least, and a spectrum analyzer at best. 
Neither instrument is often found in the homeowner's toolbox.

If the strong and the weak stations come from very different directions, 
highly directional antennas can be used, with the amplifier on only the 
weak-station one(s). The signals can be combined after they have been 
brought to more or less the same level.

Another (not very good) answer is to simply attenuate all the signals 
with a broadband attenuator until the level of the hottest signal is at 
or below the rating of the amplifier. Unfortunately this usually cancels 
the benefit of the amp.

The problem can also exist with strong local FM radio stations, even if 
all the TV signals are weak, FM stations fall within the passband of the 
amplifier and can create distortion products that fall within the TV 
bands. Some amplifiers have a tunable trap to reduce the level of a 
strong FM station, but if you have 2 or more such stations you're in 
trouble.

You can also buy an FM band trap that blocks the entire FM band, placing 
this ahead of the amplifier can help some situations.

If the local stations are UHF, and the distant ones are VHF (or the 
opposite situation) you can buy traps that attenuate the entire VHF or 
UHF band.

You can also have problems with strong local non-broadcast signals, 
examples are two-way radios, airport beacons and even amateur radio 
operators ("hams").

(BTW "hams" are generally eager to work with you if you are having 
problems caused by their signals, even when the fault is your receiving 
equipment and not their transmitters).

So, if you have weak distant signals as well as strong local ones (TV, 
FM or "other") you are going to have problems getting an amplifier to 
work well. You may want to consult with a local contractor, who may have 
traps already set up for the local situation.

It's not hopeless, just more complicated than most folks realize :-)

beww




More information about the mythtv-users mailing list